Re: [oc] New! FFT core

[John Dalton <john.dalton@bigfoot.com>]

> After me, may be John can can do a final round of editing ? ;*)

Did someone say my name?  Does 'John' refer to me?

If so, I don't mind doing an edit.  My FFT core has been at a stand still
for ages now, so if I can make a contribution by doing some editing, I'm glad to.

A corrected version of the text from the document cfft/cfft_e.pdf is appended to this email.

Regards
John

--------------------

1. Description

CFFT is a radix-4 fast Fourier transform (FFT) core with configurable data width and
a configurable number of sample points in the FFT.

Twiddle factors are implemented using the CORDIC algorithm, causing the gain of the
CFFT core to be different from the standard FFT algorithm.  This variation in gain
is not important for orthogonal frequency division modulation (OFDM) and demodulation.
The gain can be corrected, to that of a conventional FFT, by applying a constant
multiplying factor.  The input to the FFT is naturally ordered {editor's note: have I
interpreted this correctly?} while the output of the FFT is bit reversed,

2.  Theory of Operation

The reader is referred to one of the many explanations given in common textbooks.
{editor's note: provide reference}

3. Block Diagram

4. Timing

5. Miscellaneous

a. Gain

   Points   Standard FFT  CFFT      Standard IFFT  CFFT(inverse)
   256      1             0.0698    1/256          17.9
   1024     1             0.0287    1/1024         29.4
   4096     1             0.0118    1/4096         48.2742

b. Output Order

   The output sequence is bit reversed.  For example, the 789th (1100010101(base 2)) output
   of a 1024 point FFT corresponds to a normalised frequency of 675/1024 (1010100011(base 2)).

c. Synthesis

   This core can be synthesized using Synplify70.  It can be placed and routed using Xilinx ISE4.1.
   A 1024 point FFT, with a 12 bit I/Q input can be fitted into an XCV50E-6 and runs at a
   clock speed of 90MHz.


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